Ion beams have been used as probes in TOF (Time-of-Flight) mass spectroscopy of surfaces of material for years (Hammond et al., 1995). Imaging and elemental analysis by energy analysis of the backscattered ions and backscattered neutrals and forward recoiled elemental ions and neutrals created during the collisions between incident ions and surfaces can yield information on both surface structure and composition. Scanning Focused Ion Beams (FIB) and recording the backscatter intensity (backscatter ions, secondary electrons, secondary ions sputtered from the surface) have been used to image semiconductor and biological surfaces and ion mill samples under investigation.
Measuring the backscattered ions/neutrals, the electrons generated when an ion beam strikes the surface of a material has been used extensively for elemental and structural analysis of the surface for the last 25 years. The co-axial impact collision ion scattering spectrometry (CAICISS) (Katayama et al., 1988 and Aono et al., 1992) technique was developed by measuring energy losses of backscattering Helium atoms and Helium ions when a nanosecond pulsed Helium ion beam impinges a surface. The energy of the backscattered Helium is determined by measuring the time of flight from the sample to the detector. The time of flight from the sample to the detector is relative to the time at which the Helium ion beam is initially pulsed. Since the mass of Helium is known and the length from the ion source to the sample and the sample to the detector are well defined by geometry, the energy loss of each Helium atom arriving at the detector can be computed. The energy will be high (fast time of flight) when the Helium backscatters from a heavy element and low when it strikes a light element (slow time of flight). It is important to note when using CAICISS most ions neutralize as they approach the surface and remain neutralized as the ions backscatter from the surface. Because the velocity of Helium at a few hundred to a few thousand eV of kinetic energy is still large enough, the neutral helium will be detected with near unit efficiency when they impinge a channel plate detector. Thus most of the Helium which backscatters from the surface into an angle subtended by the detector can be detected
There are some limits to the elemental mass specificity of this technique. For example, light elements, such as Oxygen, are detected poorly by Helium backscatter relative to heavier elements, such as Zinc. Thus the technique may be used in conjunction with one or more detectors placed in the forward scattering direction so that the energy of light recoiled surface elements can be determined.
Another example and application of CAICISS is to monitor film growth. In such applications, elements such as Lanthanum (La) and Strontium (Sr) are difficult to resolve due to their similar masses which result in nearly equal Helium backscatter flight times. Depending on the azimuthal scattering angle, the signal intensities can vary significantly. The variation in signal intensity depends on scattering from heavy species like Lanthanum (La) or Strontium (Sr) compared to the lighter material like Manganese. Also, the variation in signal intensity depends on the surface structure (where each element is shadowing and blocking its nearest neighbor at certain angles).
The physical scale of these instruments is another drawback. The beamline and backscattering detector are over a meter in length. The actual flight path for the backscattered ions/neutrals is about 500 millimeters (mm), this path length is necessary to obtain an acceptable spectra when the pulse of Helium is tens of nanoseconds. Also, because of this geometry, the angle subtended by a 50 millimeters diameter detector, the detector is very small (less than 1 degree half angle).
When scanning a microfocused energetic primary particle beam (electron, ion, photon), spatially resolved microprobe images of the surface are routinely obtained. The microprobe images are most often obtained by measuring and recording the variation of the secondary electron yield as the particle beam is scanned from one microfocused point on the surface to the next. Also, the images may be obtained by other contrast mechanisms such as by measuring the intensity, the energy and/or the mass of secondary ejected particles using an ionizing radiation such as photon irradiation. The secondary ejected particles include but are not limited to photons, backscattered primary particles, secondary ions directly created and sputtered by the incoming primary particle beam, or secondary ions created by photoionizing sputtered neutral elements or molecules. The present invention provides a detector for correlating coincident particle emissions with spatial imaging by a laser or particle beam microprobe.